Method of growing zinc-oxide-based semiconductor and method of manufacturing semiconductor light emitting device

ABSTRACT

A method includes the steps of, using water vapor and a metalorganic compound not containing oxygen, (a) performing crystal growth at a low growth temperature and at a low growth pressure in the range of 1 kPa to 30 kPa to form a low-temperature grown single-crystal layer; and (b) performing crystal growth at a high growth temperature and at a pressure higher than the low growth pressure to form a high-temperature grown single-crystal layer on the low-temperature grown single-crystal layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of growing a zinc-oxide-basedsemiconductor and a method of manufacturing a semiconductor device, andparticularly to a method of growing a zinc-oxide-based semiconductorlayer by an MOCVD method and a method of manufacturing a semiconductorlight emitting device using the method.

2. Description of the Related Art

Zinc oxide (ZnO) is a direct transition semiconductor having band gapenergy of 3.37 eV at room temperature and expected to serve as amaterial for optical devices of a blue to ultraviolet range. Inparticular, the binding energy of excitons thereof being 60 meV withrefractive index n=2.0, ZnO has physical properties extremely suitablefor semiconductor light emitting devices. Further, not being limited tolight emitting and light receiving devices, ZnO can be widely applied tosurface acoustic wave (SAW) devices, piezoelectric devices, and thelike. Moreover, ZnO has features that its raw materials are inexpensiveand that it is harmless to the environment and human bodies.

Generally, an MOCVD (Metal Organic Chemical Vapor Deposition) method andan MBE (Molecular Beam Epitaxy) method are being used as the crystalgrowth method of a zinc-oxide-based compound semiconductor. The MBEmethod is a crystal growth method performed under an ultrahigh vacuum,and hence there is the problem that the apparatus for it is expensiveand that productivity is low. In contrast, for the MOCVD method, theapparatus is relatively inexpensive, and large area growth andmulti-wafer simultaneous growth are possible. Thus, the MOCVD method hasthe advantages of high throughput and being excellent in massproductivity and cost.

A zinc oxide crystal growth method using the MBE method wherein alow-temperature ZnO layer showing single-crystal characteristics isgrown on, e.g., a sapphire substrate and then flattened by heattreatment at a high temperature and thereafter a high-temperature ZnOlayer is grown to obtain a ZnO layer good in crystallinity is disclosedin, e.g., Japanese Patent No. 3424814 (Reference 1). More specifically,a low-temperature grown ZnO single-crystal layer formed at a growthtemperature lower than a crystal growth temperature at which a ZnOsingle crystal is generally grown, is disclosed. However, the methoddisclosed in Reference 1 is a growth condition/method effective only forthe MBE method and cannot be applied to the MOCVD method. That is, thegrowth conditions for the MBE method, where crystal growth innon-stoichiometry conditions is possible, cannot be applied, as it is,to the MOCVD method as well known (e.g., Japanese Patent ApplicationLaid-Open Publication No. 2005-340370 (Reference 2)). Hence, methods ofgrowing a single-crystal layer of a zinc-oxide-based semiconductor usingthe MOCVD method are being actively studied.

Various methods of growing zinc oxide (ZnO) or a zinc-oxide-basedsemiconductor on a substrate of another material such as sapphire(Al₂O₃) by the MOCVD method have been disclosed in, e.g., Reference 2and Japanese Patent No. 3859148 (Reference 3). For example, Reference 2discloses that micro crystals of MgZnO are formed as a preliminarybuffer layer using gas O₂ as an oxygen source on an A-plane sapphiresubstrate or a C-plane silicon carbide substrate and that with the microcrystals as seed crystals, a MgZnO crystal is formed as an actual bufferlayer entirely over the substrate. Reference 3 discloses that alow-temperature formed polycrystal or amorphous laminate is annealed ata high temperature so as to be a buffer layer. However, with thesemethods, when the polycrystal is single-crystallized by heat treatment,defects are left between adjacent crystal grain boundaries, and when theactual buffer layer is grown from the micro crystals with grainsmerging, defects are left. Hence, it is difficult to greatly reduce thenumber of crystal defects. As such, to date, attempts to improvecrystallinity after forming a buffer layer of a polycrystal, microcrystals, or amorphous material have been made, but the method toimprove crystallinity for the MOCVD method is complex, and it has beendifficult to grow a ZnO-based crystal of high crystalline quality on asubstrate.

As described above, methods of growing a ZnO-based single crystal on asapphire substrate or the like using the MOCVD method have been proposedin large number, but they fall short of being a method of growing aZnO-based single crystal of high crystalline quality in a simple,convenient way.

Further, in the case of the MOCVD method, if crystal growth is performedin the environment of a temperature at which crystallinity is improved(about 600° C. or higher), the ZnO-based single crystal tends to becomea (hexagonal) columnar crystal, a mesh-like crystal, or a (hexagonal)disc-like crystal which is oriented in a c-axis direction. With thispolycrystal or imperfect single crystal strongly oriented in a crystalaxis direction, grain boundaries and dislocations cause a leak currentor local current concentration in semiconductor devices, resulting indegradation of device characteristics and device lifetime. Inparticular, in semiconductor light emitting devices, leak currents andcurrent concentration result in degradation of characteristics such aslight-emission efficiency and device lifetime. Further, the crystalsurface being not even or flat results in a decrease in process accuracyin semiconductor processes such as lithography and etching and adecrease in production yield, and also results in a decrease inproduction yield in cleavage, breaking, and the like.

As such, to date, it has been difficult to grow a ZnO-basedsemiconductor single crystal which is flat and has a small number ofgrain boundaries and dislocations, on a substrate of another materialsuch as a sapphire substrate by the MOCVD method. In order to makesemiconductor devices, especially semiconductor light emitting devicesdriven by large operating current density, higher in performance andreliability, it is extremely important to develop a method of growing acrystal close to an ideal crystal, which has a small numbers of crystaldefects or low defect density and is flat.

SUMMARY OF THE INVENTION

The present invention was made in view of the above-described problems,and objects thereof are to provide a method of growing azinc-oxide-based semiconductor crystal which has a small number ofcrystal defects or low defect density and is excellent in single-crystalquality and flatness over a substrate, and to provide high performance,high reliability semiconductor devices, especially high performancesemiconductor light emitting devices excellent in light-emissionefficiency and device lifetime, and in addition to provide semiconductorlight emitting devices high in production yield and excellent in massproductivity.

According to the present invention, there is provided a method ofgrowing a zinc-oxide-based semiconductor layer on a substrate by anMOCVD method, comprising the steps of, using water vapor and ametalorganic compound not containing oxygen, (a) performing crystalgrowth at a low growth temperature and at a low growth pressure in therange of 1 kPa to 30 kPa to form a low-temperature grown single-crystallayer; and (b) performing crystal growth at a high growth temperatureand at a pressure higher than the low growth pressure to form ahigh-temperature grown single-crystal layer on the low-temperature grownsingle-crystal layer.

Further, according to the present invention, there is provided a methodof manufacturing a semiconductor light emitting device by growingzinc-oxide-based semiconductor layers on a substrate by an MOCVD method,comprising the steps of, using water vapor and a metalorganic compoundnot containing oxygen, (a) performing crystal growth at a low growthtemperature and at a low growth pressure in the range of 1 kPa to 30 kPato form a low-temperature grown single-crystal layer; (b) performingcrystal growth at a high growth temperature and at a pressure higherthan the low growth pressure to form a high-temperature grownsingle-crystal layer on the low-temperature grown single-crystal layer;and (c) performing crystal growth at a high growth temperature and at apressure higher than the low growth pressure to form a light emittinglayer on the high-temperature grown single-crystal layer.

The low growth temperature may be a temperature in the range of 250° C.to 450° C.

The high growth temperature may be a temperature in the range of 700° C.to 850° C.

The pressure higher than the low growth pressure may be a pressure inthe range of 40 kPa to 120 kPa.

According to another aspect of the present invention, thelow-temperature grown single-crystal layer forming step (a) includes thesteps of (a1) performing crystal growth at a first low growthtemperature and at a low growth pressure in the range of 1 kPa to 30 kPato form a first low-temperature grown single-crystal layer; and (a2)performing crystal growth at a second low growth temperature higher thanthe first low growth temperature and at a pressure higher than the lowgrowth pressure to form a second low-temperature grown single-crystallayer on the first low-temperature grown single-crystal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a low-temperature grown crystallayer and a ZnO-based semiconductor layer grown on a substrate accordingto the present invention;

FIG. 2 shows a crystal growth sequence used in a crystal growth by anMOCVD method;

FIGS. 3A and 3B show RHEED diffraction images of a ZnO layer for thecases of performing no heat treatment thereon after grown and ofperforming heat treatment, respectively;

FIG. 4 shows the full width at half maximum (FWHM) of the X-raydiffraction (10-10) ω rocking curve of a ZnO layer (a secondsingle-crystal layer) against growth pressure;

FIG. 5 shows the full width at half maximum (FWHM) of the X-raydiffraction (10-10) ω rocking curve of the ZnO layer (the secondsingle-crystal layer) against growth temperature;

FIG. 6 shows the full width at half maximum (FWHM) of the X-raydiffraction (0002) ω rocking curve of a ZnO-based semiconductor layer (ahigh-temperature grown single-crystal layer) against grown layerthickness;

FIGS. 7A, 7B show scanning electron microscope (SEM) images of the ZnOlayer (the second single-crystal layer) surface (grown layer thickness:1.0 μm) for growth pressures of 10 kPa (7A) and 80 kPa (7B)respectively;

FIGS. 8A, 8B, 8C show schematically a growth start method for theZnO-based semiconductor layer (the high-temperature grown single-crystallayer);

FIG. 9 is a schematic cross-sectional view of a semiconductor lightemitting device structure grown on a substrate according to the presentinvention;

FIG. 10 is a top plan view of a semiconductor light emitting device(LED);

FIG. 11 is a cross-sectional view of the semiconductor light emittingdevice (LED);

FIG. 12 is a cross-sectional view showing first and secondlow-temperature grown crystal layers and a ZnO-based semiconductor layergrown on the substrate according to the present invention;

FIG. 13 shows a crystal growth sequence used in crystal growth by theMOCVD method;

FIGS. 14A and 14B show RHEED diffraction images of a first ZnO layer forthe cases of performing no heat treatment thereon after grown and ofperforming heat treatment, respectively;

FIG. 15 shows a cross-sectional TEM image of a ZnO-based semiconductorlayer (a third single-crystal layer) after grown and surface SEM images(SEM-1 to SEM-4) of grown layers of different layer thicknesses;

FIG. 16 shows a crystal growth sequence of a modification of Embodiment1;

FIG. 17 shows the full width at half maximum (FWHM) of the X-raydiffraction rocking curve for the case of growing the high-temperaturegrown layer on the first low-temperature grown layer (LT1), the case ofgrowing on the second ZnO layer 11B grown in the growth period T=T9 toT10 (LT2A), and the case of growing on the second ZnO layer 11B grown inthe growth period T=T9 to T11 (LT2B);

FIG. 18 is a cross-sectional view of a semiconductor light emittingdevice structure grown on the substrate according to the presentinvention;

FIGS. 19A and 19B are respectively differential interference microscopeimages of the surface of a with-LED-operation-layer substrate producedaccording to the present invention and of the surface of the substratehaving an LED operation layer formed at a growth temperature of 680° C.;

FIG. 20 is a top plan view of a semiconductor light emitting device(LED); and

FIG. 21 is a cross-sectional view of the semiconductor light emittingdevice (LED).

DETAILED DESCRIPTION OF THE INVENTION

The method of growing a zinc-oxide-based semiconductor crystal layerexcellent in single-crystal quality and flatness over a substrate by theMOCVD method will be described in detail below with reference to theaccompanying drawings. Description will be made taking a semiconductorlight emitting device (LED: Light Emitting Diode) as an example of asemiconductor device formed by the growing method. The same referencenumerals are used to denote substantially the same or equivalentportions throughout the figures cited below.

The “low growth temperature” defined herein refers to a temperature of,e.g., about 200° C. to 650° C. that is about 50° C. to 500° C. lowerthan a crystal growth temperature at which a ZnO single crystal isgenerally grown. The “high growth temperature” refers to a temperatureof about 850° C. or lower that is a growth temperature generallysuitable for growing a ZnO single crystal and higher than the “lowgrowth temperature”.

Embodiment 1

FIG. 1 is a cross-sectional view showing a zinc-oxide-basedsemiconductor crystal layer (hereinafter, simply referred to as aZnO-based semiconductor layer) grown on a substrate 10 according to thepresent invention. More specifically, a ZnO layer 11 and a ZnO-basedsemiconductor layer 12 are grown on a sapphire substrate using the MOCVDmethod. Description will be made below taking as an example the case ofgrowing Mg_(x)Zn_((1-x))O (0≦x≦0.68) as the ZnO-based semiconductorlayer 12.

Note that the ZnO-based semiconductor may be another ZnO-based compoundcrystal, not being limited to Mg_(x)Zn_((1-x))O. For example, it may bea ZnO-based compound crystal having a portion of Zn (zinc) replaced bycalcium (Ca), or a ZnO-based compound crystal having some of O (oxygen)replaced by selenium (Se), sulfur (S), tellurium (Te), or the like.

The MOCVD apparatus (not shown) used in crystal growth has a reactioncontainer (chamber), and inside the reaction chamber, there are provideda susceptor for holding the substrate 10, a heater for heating thesusceptor, and a shower head for blowing material gas onto thesubstrate. The apparatus is provided with an exhaust pump and a pressureadjusting apparatus for adjusting the pressure inside the chamber.

A sapphire (Al₂O₃) substrate of an α-sapphire single crystal having acorundum structure is used as the substrate 10. In the embodiment, theZnO layer 11 is grown on a sapphire A-plane using the sapphire A-plane({11-20} plane) as a crystal growth plane. Hereinafter, a substrate withthe sapphire A-plane ({11-20} plane) as the main plane (i.e., crystalgrowth plane) is also called an A-plane sapphire substrate. Here, Millerindices enclosed in “{ }” indicate representative values of equivalentplanes.

In the embodiment, a material not containing oxygen was used as ametalorganic compound material (or organic metal material). That is, amaterial containing neither an oxygen atom nor an oxygen molecule in itsconstituent molecule was used.

More specifically, DMZn (dimethyl zinc) was used as a zinc (Zn) source,Cp2Mg (biscyclopentadienyl magnesium) as a magnesium (Mg) source, andTEGa (triethyl gallium) as a gallium (Ga) source. Other than thesematerials, DEZn (diethyl zinc), TMGa (trimethyl gallium), etc., can beused. The organic metal material not containing oxygen is easy topyrolytically decompose at low temperatures and reacts with O₂ (oxygen)or H₂O (water vapor) even at room temperature, hence being suitable forcrystal growth at low temperature.

H₂O (water vapor) was used as liquid material for an oxygen source. H₂Ohas high reactivity with the metalorganic material not containing oxygenin its molecule even at about room temperature, hence being suitable forthe low-temperature growth of a ZnO crystal. Further, the H₂O moleculeis large in the polarization within the molecule and has two loneelectron pairs, hence being excellent in the ability of adsorption to acrystal surface or the like.

TMGa (trimethyl gallium) was used as an n-type impurity source, and NH₃(ammonia) gas was used as a p-type impurity source. Note that thesegases may be diluted with inert gas such as nitrogen or Ar (argon).

Further, nitrogen was used as carrier gas. The vapor of a liquid orsolid material and gas material (hereinafter called material gas) areconveyed into the shower head by the carrier gas and supplied to thesubstrate.

FIG. 2 shows a crystal growth sequence used in a crystal growth by theMOCVD method. First, the A-plane sapphire substrate 10 was set on thesusceptor in the MOCVD apparatus, and the reaction chamber pressure wasadjusted to 10 kPa (kilopascals) (time T=T1).

Then, H₂O (water vapor) as an oxygen source started to be supplied fromthe shower head to the substrate 10 at a flow rate of 640 μmol/min(T=T2). Further, the substrate was raised in temperature by the heaterfrom room temperature (RT) to 900° C., and the degassing of thesubstrate 10 was performed for 10 minutes (T=T3 to T4). Thereafter, H₂O(water vapor) was kept flowing at the same flow rate until the growthfinished.

After the degassing, the substrate temperature was lowered to 400° C.,and DMZn (dimethyl zinc) as zinc material was supplied to the substrate10 at a flow rate of 1 μmol/min for about 15 minutes (T=T5 to T6: thegrowth period of the ZnO layer 11 indicated by period G1A in FIG. 2). Atthis time, the ratio of the water vapor flow rate to the organic metal(DMZn) flow rate (F_(H2O)/F_(DMZn) ratio), a so-called VI/II ratio, wasat 640.

As such, the ZnO layer (low-temperature grown single-crystal layer,hereinafter also simply referred to as a first single-crystal layer) 11of 25 nm thickness was grown under a reduced pressure growth condition(i.e., growth pressure Pg=10 kPa) at a growth temperature (Tg) of 400°C. at a growth rate of 1.7 nm/min.

After the growth of the ZnO layer 11, the substrate temperature wasraised to 800° C. to perform heat treatment (annealing) for 10 minutes(T=T7 to T8). As described later, the crystallinity and flatness of theZnO layer 11 were further improved by the heat treatment.

After the heat treatment (annealing) finished, the ZnO-basedsemiconductor layer 12 was grown at a higher temperature and a higherpressure than those of the ZnO layer (i.e., low-temperature grownsingle-crystal layer) 11. The reaction chamber pressure was increasedfrom 10 kPa to 80 kPa. After the substrate temperature became stable,DMZn was supplied from the shower head onto the ZnO layer 11 at a flowrate of 10 μmol/min (T=T9 to T10: the growth period of the ZnO-basedsemiconductor layer 12 indicated by the period G2 in FIG. 2). At thistime, the ratio of the water vapor flow rate to the organic metal (DMZn)flow rate (F_(H2O)/F_(MO) ratio), the so-called VI/II ratio, was at 64.DMZn was supplied to the substrate 10 for about 60 minutes to form theZnO-based semiconductor layer (ZnO layer) 12 about 1 μm thick.

Note that the standby time until the substrate temperature becomesstable before the growth of the ZnO-based semiconductor layer 12 can beshared with the heat treatment of the ZnO layer 11.

As such, the ZnO-based semiconductor layer 12 (high-temperature grownsingle-crystal layer, hereinafter also simply called a secondsingle-crystal layer) about 1 μm thick was grown at a higher pressure(80 kPa) than the growth pressure of the ZnO layer 11 at growthtemperature (Tg) as a high growth temperature (800° C.), at a growthrate of 17 nm/min. In the embodiment, description is made taking as anexample the case of growing ZnO as the ZnO-based semiconductor layer 12,but not being limited to this, a Mg_(x)Zn_((1-x))O crystal, a ternarycrystal, can be grown by using, e.g., Cp2Mg (biscyclopentadienylmagnesium) that is a metalorganic compound not containing an oxygen atomeither, as well as DMZn.

After the growth of the ZnO-based semiconductor layer 12 finished, thesubstrate was cooled with keeping water vapor flowing. After thesubstrate temperature became 300° C. or lower, the supply of the watervapor was stopped (T=T11).

As described above, a with-crystal-grown-layer substrate (hereinaftersimply called a with-grown-layer substrate) 15 that is the substratehaving the ZnO layer (first single-crystal layer) 11 and the ZnO-basedsemiconductor layer (second single-crystal layer) 12 grown thereon wasmanufactured.

[Crystallinity of the ZnO Layer (Low-Temperature Grown Single-CrystalLayer or First Single-Crystal Layer) 11]

The ZnO layer 11 obtained through the above growth process (T=T5 to T6)is a single-crystal layer c-axis-oriented on the A-plane ({11-20} plane)sapphire substrate. That is, the {0001} plane of a zinc oxide singlecrystal is laid on a substrate of an α-sapphire single crystal with the{11-20} plane as the main plane. The single-crystal quality and flatnessof the ZnO layer 11 were examined by RHEED (reflection high energyelectron diffraction) measurement and AFM (atomic force microscope)measurement.

FIG. 3A shows an RHEED diffraction image of the ZnO layer 11 without theheat treatment (anneal) after the growth process (T=T5 to T6) finished,that is, just after the growth. As shown in FIG. 3A, an RHEEDdiffraction image having streaks located at equal intervals wasobtained. From this image, it was seen that the surface crystalarrangement is a single-crystal arrangement and is flat. Also, it wasseen from AFM (atomic force microscope) measurement that Rms(root-mean-square roughness) for a 1 μm² area is 0.62 nm (nanometer).Since the c-axis length (lattice constant) c of a ZnO crystal is 5.207Å, surface roughness is comparable to the c-axis length of the ZnOcrystal. Hence it was seen that the crystal layer surface is flat. Fromthese results, it was confirmed that the ZnO layer 11 is asingle-crystal layer good in flatness.

FIG. 3B shows an RHEED diffraction image of the ZnO layer 11 on whichheat treatment was performed at a temperature of 800° C. for threeminutes after the growth. In the present embodiment, pressure Pa at theheat treatment was a low pressure (the same as at the crystal growth;Pa=Pg=10 kPa). Further, as mentioned above, while H₂O (water vapor) wasbeing supplied to the substrate 10, that is, in a water vaporatmosphere, the heat treatment was performed.

As shown in FIG. 3B, it was ascertained that the RHEED diffraction imageobtained after the heat treatment has further clear streaks than thatobtained without the heat treatment (FIG. 3A). Also, it was seen fromAFM measurement that Rms (root-mean-square roughness) is 0.5 nm or lessand is less than or equal to the c-axis length of the ZnO crystal. Thus,it was confirmed that the single-crystal quality and flatness wereimproved by the heat treatment. Namely, the first single-crystal layer11 is a thin film, but is a low-temperature grown single-crystal layerwith flatness of the order of sub-nanometers.

As such, it was confirmed that the ZnO layer 11 already has goodflatness and single-crystal quality after the crystal growth finishesand that the single-crystal quality is further improved by the heattreatment with the layer having flatness of less than or equal to thec-axis length of the ZnO crystal.

Although description has been made taking as an example the case ofgrowing the ZnO layer as the first single-crystal layer 11, it may beanother ZnO-based compound crystal.

[Growth Conditions for the Low-Temperature Grown Single-Crystal Layer(First Single-Crystal Layer) 11]

In the present embodiment, description has been made taking as anexample the case of growing the ZnO layer (first single-crystal layer)11 using an A-plane sapphire substrate, where the A-plane is the {11-20}plane of α-sapphire (α-Al₂O₃), as the substrate 10 and DMZn as themetalorganic compound material not containing oxygen, at a growthtemperature of 400° C. at a growth pressure of 10 kPa at a growth rateof 1.7 nm/min, to be 25 nm thick.

However, conditions of the substrate, material gas, growth temperature,growth pressure, growth rate, grown-layer thickness, etc., cited in thepresent embodiment are shown only as an example, not being limited tothese. The ZnO layer 11 was grown under various growth conditionsincluding heat treatment conditions, and conditions for growing asingle-crystal layer high in flatness were examined. Growth conditionsof the ZnO layer (first single-crystal layer) 11 will be described indetail below.

<Substrate>

The A-plane sapphire substrate is most suitable. This is because thelattice mismatch between the A-plane of a sapphire crystal and theC-plane of a ZnO crystal is small. That is, the lattice mismatch betweensapphire <0001> direction (oxygen atoms) and ZnO <11-20> direction (zincatoms) is 0.07%, and the lattice mismatch between sapphire <10-10>direction (oxygen atoms) and ZnO <10-10> direction (zinc atoms) is2.46%. Here, because the ZnO <11-20> direction is locked in alignmentwith the sapphire <0001> direction, a ZnO single crystal can be grownwithout 30° rotated domains being formed in the growth.

Further, as to usable substrates, for the ZnO crystal c-axis oriented ona substrate surface, a crystal substrate of which the substrate surfacehas lattice matching points at rotationally symmetric positions of 60°,120°, and 180° is appropriate, and for the ZnO crystal a-axis or m-axisoriented, a crystal substrate where a lattice matching point exists at arotationally symmetric position of 180° is appropriate. For example, anR-plane sapphire substrate, an M-plane sapphire substrate, a SiC(silicon carbide) substrate, a GaN (gallium nitride) substrate, a Ga₂O₃(gallium oxide) substrate, a Si (silicon) substrate, or the like can beused.

<Growth Temperature>

It is suitable that the growth temperature is a temperature (called a“low growth temperature”) lower than a crystal growth temperature(called a “high growth temperature”) at which a ZnO single crystal isgenerally grown. This is because at the high growth temperature ZnOtends to grow like islands and not grow into a single crystal in theform of a layer.

More specifically, the growth temperature is preferably in the range of250° C. to 450° C., more preferably in the range of 300° C. to 400° C.At temperatures lower than 250° C., ZnO tends to become amorphous orpolycrystalline since its migration length is short. Further, asmentioned above, at temperatures higher than 450° C., ZnO tends to growlike islands, and thus flatness tends to decrease (the Rms value becomeslarger).

<Growth Pressure>

The larger migration lengths of reactive chemical species (DMZn, H₂O,intermediate products, Zn atoms before crystallized, O atoms beforecrystallized, and the like) on the substrate surface are preferable forthe single crystal growth. Thus, the reduced pressure growth issuitable. Specifically, a pressure of 1 kPa to 30 kPa is suitable, and apressure of 5 kPa to 20 kPa is more preferable. If the growth pressureis 1 kPa or lower, the growth rate is significantly slow.

<Material Gas>

Forming a ZnO single-crystal layer under low growth pressure and lowgrowth temperature requires the selection of highly interactive orreactive materials. This is because, if the reactivity is low, the layerfails to crystal-grow, or becomes amorphous or polycrystalline. As theZn source, a metalorganic compound not containing oxygen in itsconstituent molecule and highly reactive with oxygen source material issuitable. Other than DMZn mentioned above, for example, DEZn (diethylzinc) may be used. As the oxygen source, H₂O (water vapor) large in thepolarization within the molecule and having high reactivity with themetalorganic compound material is suitable.

<Growth Rate>

The growth rate is preferably in the range of 0.4 nm/min to 9 nm/min,more preferably in the range of 0.8 nm/min to 4 nm/min. If the growthrate is larger than 9 nm/min, the surface bumps/dips or asperities maybecome large, resulting in insufficient flatness.

<Grown Layer Thickness>

The grown layer thickness should be in the range of 5 nm to 60 nm,preferably in the range of 10 nm to 40 nm, more preferably from 15 nm to30 nm. That is, if the layer thickness is less than 5 nm, the crystallayer may fail to sufficiently cover the substrate surface. If the layerthickness is greater than 60 nm, the surface asperities may becomelarge, resulting in insufficient flatness.

<VI/II Ratio (F_(H2O)/F_(MO) Ratio)>

The ratio of the water vapor flow rate to the organic metal (DMZn) flowrate (F_(H2O)/F_(DMZn) ratio) need only be about two or greater.Specifically, the ratio of about 2,000 will suffice. The water vaporflow rate should be up to about 70% of the saturation water vapor amountat which water vapor does not precipitate in the shower head.

<Heat Treatment Conditions>

As described above, the ZnO layer 11 already has good flatness andsingle-crystal quality after the crystal growth finishes, and thesingle-crystal quality and flatness can be further improved by the heattreatment. The heat treatment should be performed under low pressurethat makes the migration length longer. The pressure range suitable forthe heat treatment is the same as that of the growth pressure of the ZnOlayer 11.

Specifically, it is suitable that the heat treatment temperature for thegrown ZnO layer 11 is from 700° C. to 1,100° C. It is suitable thattreatment time is from 1 to 60 minutes. It is more preferable that thetreatment temperature is from 800° C. to 1,000° C. and that thetreatment time is from 3 to 10 minutes. If the treatment temperature islower than 700° C., the effect is too low, and if 1,100° C. or higher,the layer surface becomes rough. If the treatment time is 60 minutes orlonger, film deficient portions may occur due to film vaporization.

[Growth Conditions of the ZnO-Based Semiconductor Layer(High-Temperature Grown Single-Crystal Layer or Second Single-CrystalLayer) 12]

The ZnO-based semiconductor layer (second single-crystal layer) 12 isgrown on the ZnO layer (first single-crystal layer) 11 with flatness ofthe order of sub-nanometer and excellent in single-crystal quality usingconditions for growth in a two-dimensional crystal growth mode (lateralgrowth mode).

In the present embodiment, description has been made taking as anexample the case of growing the Mg_(x)Zn_((1-x))O crystal layer (secondcrystal layer) 12 on the ZnO layer 11 using water vapor and metalorganiccompound material (DMZn) not containing oxygen, at the high growthtemperature (800° C.) at a growth pressure of 80 kPa higher than that ofthe ZnO layer 11 at a growth rate of 17 nm/min, to be 1 μm thick.

However, conditions for growing the ZnO-based semiconductor layer 12 onthe ZnO layer 11 cited in the present embodiment are shown only as anexample, not being limited to these. The ZnO-based semiconductor layerwas grown under various growth conditions, and conditions for growingthe ZnO-based semiconductor layer 12 excellent in flatness andsingle-crystal quality were examined. As a result, it was found that theabove ZnO layer 11 has slight but non-negligible fluctuation incrystallinity of a certain kind and that thus there are optimumconditions for crystal growth on the ZnO layer 11. Growth conditions forgrowing the ZnO-based semiconductor layer (second single-crystal layer)12 on the ZnO layer (first single-crystal layer) 11 will be described indetail below.

<Growth Pressure>

The relationship between the growth pressure and the crystallinity andflatness of the grown layer was evaluated. From the full width at halfmaximum (FWHM) (FIG. 4) of the X-ray diffraction (10-10) ω rocking curveof the ZnO layer (second single-crystal layer) 12 for the grown layerhaving a thickness (TH) of 0.125 μm, it was found that as the growthpressure increases, the crystallinity is improved. Also, it was foundthat as the growth pressure increases, the lateral growth (i.e.,two-dimensional growth mode) is promoted, resulting in the plane(C-plane) orthogonal to the c-axis being flat. It was found that inorder to obtain a flat crystal growth surface free of bumps/dips,asperities or pits as the surface of the ZnO-based semiconductor layer(second single-crystal layer) with good crystallinity, high growthpressure is desirable. Specifically, the growth pressure should be 40kPa or higher, preferably 60 kPa or higher, more preferably 80 kPa orhigher. The upper limit should be about 120 kPa. Note that this upperlimit is that from the air tightness of the MOCVD apparatus, not fromfilm forming conditions.

<Growth Temperature>

The relationship between the growth temperature and the crystallinityand flatness of the grown layer was evaluated. It was found that as thegrowth temperature increases, the full width at half maximum (FWHM) ofthe X-ray diffraction (10-10) ω rocking curve of the ZnO layer 12 havingthe grown layer thickness (TH) of 0.125 μm becomes narrower (FIG. 5),and that dislocation density decreases, thus improving the crystallinityand flatness. Specifically, the growth temperature should be 700° C. orhigher at which a flat surface is formed in a c-axis direction. Theupper limit is about 850° C. at which the layer can hardly grow withusing H₂O (water vapor). The growth temperature is preferably in therange of 740° C. to 810° C., most preferably in the range of 780° C. to810° C.

<Grown Layer Thickness>

The relationship between the grown layer thickness and the crystallinityand flatness of the grown layer was evaluated. FIG. 6 shows the fullwidth at half maximum (FWHM) of the X-ray diffraction (0002) ω rockingcurve of the grown layer against the grown layer thickness. TheZnO-based semiconductor layer (second single-crystal layer) 12 that isformed on the ZnO layer (first single-crystal layer) 11 first grows likeislands, and as time passes, single crystal islands merge into theZnO-based semiconductor layer in the form of a layer.

FIGS. 7A, 7B show scanning electron microscope (SEM) images of the grownlayer surface of the ZnO layer 12 for growth pressures (Pg) of 10 kPa(FIG. 7A) and 80 kPa (FIG. 7B) respectively with the grown layerthickness being 1.0 μm. It can be seen that a crystal layer having goodsurface morphology and excellent in flatness is obtained when crystalgrowth is performed at a growth pressure of 80 kPa. Further, accordingto the X-ray diffraction rocking curve and scanning electron microscope(SEM) evaluations, it was found that the layer thickness is morepreferably 1.5 μm or greater in order to obtain a crystal surface havingextremely good single-crystal quality and flatness.

In order to grow a thick film having thickness of the micron ordernecessary for the device manufacture, a method is preferable wherecrystallinity tends to get better as the growth process proceeds. Inthis regard, it is understood, from FIGS. 6 and 7A, 7B, that the presentinvention is an excellent two-dimensional mode crystal growth methodwhere crystallinity and flatness both get better as the growth proceeds.

<Growth Rate>

The growth rate is preferably in the range of 5 nm/min to 60 nm/min. Ifthe growth rate is 60 nm/min or greater, abnormal growth is likely tooccur.

<Crystal Composition>

For example, a Mg_(x)Zn_((1-x))O (0≦x≦0.43) crystal can be used as theZnO-based semiconductor layer 12. However, as Mg composition xincreases, the difference in lattice constant in an a-axis directionincreases, resulting in an increase in the defect density of the grownsemiconductor crystal layer. Hence, it is more preferable that x is setas 0≦x≦0.3.

As mentioned above, the ZnO-based semiconductor layer 12 may be anotherZnO-based compound crystal. For example, it may be a ZnO-based compoundcrystal having some of Zn (zinc) replaced by Ca, or a ZnO-based compoundcrystal having some of O (oxygen) replaced by Se, S, Te, or the like.

<Material Gas>

When grown at a high temperature, a ZnO crystal of high quality can begrown because the migration lengths of reactive chemical species on thecrystal growth surface are sufficient. On the other hand, at a hightemperature, gas material that is an oxygen source becomes hard toadsorb to the substrate surface, thus hindering the growth. As theoxygen source, H₂O (water vapor) that has large polarization within themolecule, thus adsorbing even at high temperatures to the substratesurface is suitable.

As the Zn source, a metalorganic compound not containing oxygen andhighly reactive with oxygen source material is suitable. Other than DMZnmentioned above, for example, DEZn (diethyl zinc) may be used. As the Mgsource, Cp2Mg (biscyclopentadienyl magnesium) can be used.

<Growth Start Method>

FIGS. 8A, 8B, 8C show schematically a growth start method for theZnO-based semiconductor layer 12. FIG. 8A shows the case where thegrowth is performed with keeping the DMZn flow rate F_(DMZn) constantfrom the growth start time (T=T9 in FIG. 2) to the growth end time(T=T10 in FIG. 2). Here, the case of the DMZn flow rate being 10μmol/min is shown as an example.

FIG. 8B shows the case where the growth is performed with increasing theDMZn flow rate stepwise to 10 μmol/min. Specifically, the flow rate isat 3 μmol/min for 5 minutes, then at 6 μmol/min for 5 minutes, finallyat 10 μmol/min to form the ZnO-based semiconductor layer (ZnO layer) 12about 1 μm thick as in the case of FIG. 8A.

FIG. 8C shows the case where the growth is performed with increasing theDMZn flow rate linearly to 10 μmol/min in 10 minutes from the growthstart and then keeping the flow rate constant. Also in this case, theZnO-based semiconductor layer (ZnO layer) 12 about 1 μm thick is formed.Note that in the case of growing a crystal such as Mg_(x)Zn_((1-x))O,the flow rate of organic metal gas (Cp2Mg, etc.) should be changed inthe same way as the DMZn flow rate.

By increasing the DMZn flow rate stepwise or linearly in the beginningof the growth, the density of island-like crystals in the growth initialstage in the ZnO-based semiconductor layer 12 can be reduced. As aresult, there is the effect that the flat thick film of the grown layercan be made thinner (for example, 1.5 μm or less).

<Dopants>

In order to adjust the conductivity type (n-type) of the ZnO-basedsemiconductor layer 12, one or more of TMGa (trimethyl gallium), TEGa(triethyl gallium), TMAl (trimethyl aluminum), TMIn (trimethyl indium),and TEIn (triethyl indium) need to be added. However, if added beforeisland-like crystals merge and the crystal plane (C-plane) perpendicularto the c-axis becomes flat, they may hinder the surface from becomingflat, and hence, they are preferably doped during the last half of thegrowth time period in which the crystal surface becomes flat.Specifically, they are more preferably doped after the layer grows 1.0μm thick. That is, it is preferable that the ZnO-based semiconductorlayer 12 is an undoped layer at least 1.0 μm thick.

As described above in detail, by using the above growth conditions, theZnO-based semiconductor layer 12 having high flatness and crystallinitycan be grown on the first single-crystal layer 11 that is thin butexcellent in flatness and single-crystal quality. That is, the ZnO-basedsemiconductor layer (second single-crystal layer) 12 can be grown tohave enough thickness to be applied to device manufacture with havingfurther high flatness and crystallinity, and hence can be applied widelyto device manufacture. For example, a light emitting layer (or activelayer), an LED light emitting operation layer, a clad layer ofsemiconductor laser devices, or a device operation layer of electronicdevices or the like can be grown directly on the ZnO-based semiconductorlayer 12. Or, the ZnO-based semiconductor layer 12 grown under the abovegrowth conditions may be arranged to form part of an LED light emittingoperation layer, a clad layer, or a device operation layer.

In this specification, the operation layer or device operation layerrefers to a layer constituted by a semiconductor which a semiconductordevice must include to fulfill its function. For example, for a simpletransistor, a structured layer constituted by pn junctions of an n-typesemiconductor, a p-type semiconductor, and an n-type semiconductor (or ap-type semiconductor, an n-type semiconductor, and a p-typesemiconductor) are included. A semiconductor layer constituted by ap-type semiconductor layer, a light emitting layer, and an n-typesemiconductor layer (or a p-type semiconductor layer and an n-typesemiconductor layer) and emitting light through the recombination ofinjected carriers is called a light emitting operation layer.

[With-Grown-Layer Substrate 15]

The with-grown-layer substrate 15 obtained through the above processescan be subsequently used in the MOCVD apparatus to manufacture asemiconductor device without being cooled. Or, after cooled,semiconductor devices may be manufactured using the MOCVD apparatus oranother crystal growing apparatus.

That is, with the with-grown-layer substrate 15, a single crystal can begrown directly on the single-crystal layer (ZnO-based semiconductorlayer 12) excellent in single-crystal quality and flatness using theMOCVD apparatus or another crystal growing apparatus. Thus, a highquality ZnO-based semiconductor layer having a small numbers of crystaldefects and low defect density, and excellent in single-crystal qualityand flatness can be formed.

Further, with the with-grown-layer substrate 15, optical semiconductordevices, various electronic devices, and the like can be formed byvarious methods other than MOCVD such as MBE, plasma CVD, PLD (PulsedLaser Deposition), and hydride VPE.

As such, according to the present invention, there is provided awith-grown-layer substrate having formed thereon high qualitysingle-crystal layers having a small number of crystal defects andexcellent in single-crystal quality and flatness which is applicable tothe manufacture of optical semiconductor devices and various electronicdevices.

Embodiment 2

FIG. 9 is a cross-sectional view of a semiconductor light emittingdevice structure grown on the substrate 10 according to the presentinvention. More specifically, a light emitting operation layer(hereinafter also called an LED operation layer) 20 including an n-typeZnO-based semiconductor layer 21, a ZnO-based semiconductor active layer22, and a p-type ZnO-based semiconductor layer 23 was formed on thewith-grown-layer substrate 15 having the ZnO layer 11 and the ZnO-basedsemiconductor layer 12 formed thereon according to the above-describedembodiment. Crystal growth conditions of the LED operation layer 20 werethe same as those of the ZnO-based semiconductor layer 12 unlessotherwise noted. That is, the crystal growth sequence was the same asthat shown in FIG. 2, and using the same growth temperature, growthpressure, material gas, etc., as in the period (G2) of T=T9 to T10, then-type ZnO-based semiconductor layer 21, ZnO-based semiconductor activelayer 22, and p-type ZnO-based semiconductor layer 23 were sequentiallyformed on the ZnO-based semiconductor layer 12. Growth conditions suchas growth temperature and growth pressure of the LED operation layer 20may not necessarily be the same as those of the ZnO-based semiconductorlayer 12. That is, crystal growth conditions of the LED operation layer20 are preferably within the ranges of those of the ZnO-basedsemiconductor layer 12.

As such, by growing an LED operation layer on the ZnO-basedsemiconductor layer 12 having good flatness and single-crystal qualityunder growth conditions similar to those of the ZnO-based semiconductorlayer 12, the LED operation layer flat and excellent in crystallinitycan be formed. Description will be made below taking as an example thecase of growing Mg_(x)Zn_((1-x))O as the ZnO-based semiconductor.

After an undoped ZnO-based semiconductor layer 12 was grown, withmaintaining the growth temperature and growth pressure (800° C., 80 kPa)the n-type ZnO-based semiconductor layer 21 was grown. The undopedZnO-based semiconductor layer 12 is preferably at least 1.0 μm thick asmentioned above.

Keeping the H₂O (water vapor) flow rate at 640 μmol/min and increasingthe DMZn flow rate from 10 μmol/min to 30 μmol/min, a Ga-dopedMg_(x)Zn_((1-x))O crystal 3 μm thick was grown. The flow rate of Mgmaterial gas (Cp2Mg) should be adjusted according to Mg crystalcomposition x.

In order to control the conductivity type (n-type), TEGa was dopedduring the Mg_(x)Zn_((1-x))O crystal growth so that its concentration inthe Mg_(x)Zn_((1-x))O crystal became 5×10¹⁸ (cm⁻³).

Then, the DMZn flow rate was decreased to 1 μmol/min, and the activelayer 22 (Mg_(x)Zn_((1-x))O crystal) 30 nm thick was grown. Here, bydecreasing the DMZn flow rate, the ratio of the H₂O (water vapor) flowrate to the DMZn flow rate (VI/II ratio) was increased from 21 to 640.Thereby, deficiency of oxygen or the like in the grown layer could bereduced, and thus high light-emission efficiency could be obtained.

Then, the p-type ZnO-based semiconductor layer 23 was grown.Specifically, at a DMZn flow rate of 1 μmol/min, an N (nitrogen)-dopedMg_(x)Zn_((1-x))O crystal 100 nm thick was grown. Here, during theMg_(x)Zn_((1-x))O crystal growth, NH₃ (ammonia) was supplied as p-typeimpurity material (dopant) at a flow rate of 180 μmol/min so thatnitrogen impurity concentration Na (N) became 8×10¹⁹ cm⁻³.

After the above process finished, the pressure was maintained at 80 kPawith keeping water vapor flowing until the substrate temperature became300° C. After the substrate temperature became 300° C. or lower, watervapor was stopped, and the substrate was taken out of the reactionchamber after the substrate temperature became room temperature.

The case of using Mg_(x)Zn_((1-x))O crystals as the n-type ZnO-basedsemiconductor layer 21, active layer 22, and p-type ZnO-basedsemiconductor layer 23 has been described.

In this case,n-type ZnO-based semiconductor layer 21: Mg_(x)Zn_((1-x))O (0≦x≦0.43);p-type ZnO-based semiconductor layer 23: Mg_(x)Zn_((1-x))O (0≦x≦0.43);andactive layer 22: Mg_(x)Zn_((1-x))O/Mg_(y)Zn_((1-y))O (0≦(x, y)≦0.43;y<x) can be used.

Here, Mg crystal composition x should be 0.63 or less for a layerthickness of about 0.5 μm or less and 0.43 or less for 0.5 μm orgreater. This is because if the Mg composition is higher, the phaseseparation of MgO occurs in the MgZnO crystal.

Each of the n-type ZnO-based semiconductor layer 21, the active layer22, and the p-type ZnO-based semiconductor layer 23 may be of amultilayered structure depending on the light emitting devicecharacteristics. Further, the active layer 22 may be of an MQW(Multi-quantum well) structure. Especially in the case of an MQW activelayer, variation in the thickness of crystal layers (a well layer, abarrier layer) varies quantum state energy, quantum state density, andthe like, thus greatly affecting the emission wavelength, internalquantum efficiency, and the like, and hence the effect produced by usingthe ZnO-based semiconductor layer 12 excellent in flatness andsingle-crystal quality is further remarkable.

A with-LED-operation-layer substrate 25 that was manufactured throughthe above-described process was evaluated with a differentialinterference microscope and a scanning electron microscope (SEM). As aresult, it was ascertained that the surface was free of bumps/dips orpits and was an extremely flat and mirror-like in macroscopic tomicroscopic views.

[Manufacture of Semiconductor Light Emitting Device]

Semiconductor light emitting devices (LEDs) were manufactured using thewith-LED-operation-layer substrate 25 produced through the aboveprocess, through the following processes. FIG. 10 is a top plan view ofa semiconductor light emitting device (LED) 30, and FIG. 11 is across-sectional view of LEDs 30. FIG. 11 shows that two LEDs 30 and adevice partition groove 32 for separating these by breaking process areformed.

First, a resist mask of a shape covering an area (i.e., device area) ina device section 31 was formed using photolithography technology. Then,portions of the p-type ZnO-based semiconductor layer 23, the activelayer 22, and the n-type ZnO-based semiconductor layer 21 under theopening outside the device section 31 were removed and etched down to apredetermined depth using wet etching. Finally, the resist was removedto form the device partition groove 32.

The surface of the with-LED-operation-layer substrate 25 of the presentinvention is flat and mirror-like entirely on the substrate (e.g., a twoinch substrate), and hence a resist can be coated to have uniformthickness. Further, because the surface is free of bumps/dips, in thephotolithography process, a pattern can be transferred with goodaccuracy without exposure halation of the transferred pattern.

Then, a p-side electrode 33 was formed using photolithography, EB(electron beam) vapor deposition, and so on. As to the p-side electrode33, a Ni (nickel) film 0.3 to 10 nm thick and an Au (gold) film 5 to 20nm thick were formed, and a process at 500° C. for 30 seconds wasperformed in a 10% oxygen atmosphere in an RTA (rapid thermal annealer)to make it a transparent electrode.

Next, Ni, Pt, and Au electrode pad materials were laid one over anotherin that order to be respectively 3 to 10 nm, 100 nm, and 1,000 nm thickon the p-side electrode 33 to form a p-side connection electrode 34.

Further, Ti and Au were laid one over the other in that order by EBvapor deposition to be respectively 10 to 100 nm and 1,000 nm thick on asurface of the n-type ZnO-based semiconductor layer 21 exposed byphotolithography to form an n-side connection electrode 35.

After the electrodes were formed in this way, the substrate 25 was stuckat the LED operation layer side to a protection substrate, and the backside was grinded and polished to be shaped into a wafer 100 μm thick.Next, after a protection sheet was stuck to the electrode-formed surfaceside, scribe grooves 36 orthogonal to each other were formed in the backside to be opposite the center of the device partition grooves 32 with ascribe apparatus.

In the breaking process, a knife edge 37 was pressed against the bottomof the device partition groove 32 opposite the scribe groove 36, and aload was applied with the knife edge 37 to cleave the substrate along ascribe groove. Likewise, the substrate was rotated through 90° andcleaved along the scribe groove 36 orthogonal to the former one.

Because the surface of the with-LED-operation-layer substrate 25 of thepresent invention was free of bumps/dips or asperities, the bottom ofthe device partition groove 32 was also formed extremely flat, andpressure could be accurately applied to the partition groove bottom withthe knife edge 37. Thus, when applying pressure to cleave, stress wasuniformly applied, and hence the occurrences of cleavage failure due todevice section chipping, the deviation of a cleaved surface, etc., couldbe reduced, improving the cleavage yield. Further, without a potentialcrystal grain boundary (domain) in crystal planes, when cleaving, chipsin a crystal layer due to a grain boundary decreased in number,improving the separation yield.

When devices were formed using the with-LED-operation-layer substrate 25of the present invention, a resist pattern formation yield was about98%. Also, the cleavage yield was extremely good, about 98%.

As described above, because an LED operation layer is grown on theZnO-based semiconductor layer 12 that is a high quality single crystallayer having a small number of crystal defects and excellent insingle-crystal quality and flatness, the LED operation layer flat andexcellent in crystallinity can be formed, and high performance LEDshaving leak current and current concentration suppressed and thus highin light-emission efficiency and excellent in device lifetime can bemanufactured. Further, high process accuracy in semiconductor processessuch as lithography and etching can be obtained, and the productionyield in cleavage, breaking, and like is also high.

As described above in detail, according to the present invention, usingmetalorganic compound material not containing oxygen together with H₂O(water vapor) having high reactivity with the metalorganic compoundmaterial, a zinc oxide single-crystal layer (first single-crystal layer)can be grown at a low growth temperature and a low growth pressure on asubstrate of another material substrate such as an A-plane sapphiresubstrate. Further, by performing heat treatment under low pressure(reduced pressure) in a water vapor atmosphere, the single-crystalquality and flatness of the grown layer can be further improved. Thesingle-crystal layer is excellent in flatness and single-crystal qualityand also has a small number of crystal defects. Moreover, because theMOCVD method is used, large area growth and multi-wafer growth arepossible. Thus, the present method is excellent in mass productivity andproduction costs.

Furthermore, the ZnO-based compound semiconductor layer having furtherhigh flatness and crystallinity can be grown on the ZnO layer (firstsingle-crystal layer) 11 excellent in flatness and single-crystalquality. The ZnO-based compound semiconductor layer (secondsingle-crystal layer) 12 can be grown to have enough thickness to beapplied to device manufacture as well as high flatness andcrystallinity. Therefore, an LED light emitting operation layer, a cladlayer of semiconductor laser devices, a device layer of electronicdevices, or the like can be grown directly on the ZnO-basedsemiconductor layer 12.

By growing an LED operation layer on the ZnO-based semiconductor layer12, the LED operation layer flat and excellent in crystallinity can beformed, and high output power, high performance LEDs having leak currentand current concentration suppressed and thus high in light-emissionefficiency can be provided. Further, LEDs excellent in device lifetimecan be manufactured. Yet further, high accuracy in semiconductorprocesses is obtained, and the production yield in cleavage, breaking,and like is also high.

Embodiment 3

FIG. 12 is a cross-sectional view of a zinc-oxide-based semiconductorcrystal layer grown on the substrate 10 according to the presentinvention. More specifically, a first ZnO layer 11A, a second ZnO layer11B, and a ZnO-based semiconductor layer 12 are formed on a sapphiresubstrate using the MOCVD method. Description will be made below takingas an example the case of growing Mg_(x)Zn_((1-x))O (0≦x≦0.68) as theZnO-based semiconductor layer 12.

FIG. 13 shows a crystal growth sequence used in crystal growth by theMOCVD method. First, the A-plane sapphire substrate 10 was set on thesusceptor in the MOCVD apparatus, and the reaction chamber pressure wasadjusted to 10 kPa (kilopascals) (time T=T1).

Next, H₂O (water vapor) as an oxygen source started to be supplied fromthe shower head to the substrate 10 at a flow rate of 640 μmol/min(T=T2). Further, the substrate 10 was raised in temperature by theheater from room temperature (RT) to 900° C. to perform the degassing ofthe substrate 10 for 10 minutes (T=T3 to T4). Thereafter, H₂O (watervapor) was kept flowing at the same flow rate until the growth finished.

After the degassing, the substrate temperature was lowered to 400° C.(first low growth temperature), and DMZn (dimethyl zinc) as zincmaterial was supplied to the substrate 10 at a flow rate of 1 μmol/minfor about 15 minutes (T=T5 to T6: the growth period of the first ZnOlayer 11A indicated by period G1A in FIG. 13). At this time, the ratioof the water vapor flow rate to the organic metal (DMZn) flow rate(F_(H2O)/F_(DMZn) ratio), a so-called VI/II ratio, was at 640.

As such, the ZnO layer (first low-temperature grown single-crystallayer, hereinafter also simply called a first single-crystal layer) 11Aof 25 nm thickness was grown under a reduced pressure growth condition(i.e., growth pressure Pg=10 kPa) at a growth temperature (Tg) of 400°C. at a growth rate of 1.7 nm/min.

After the growth of the ZnO layer 11A, the substrate temperature wasraised to 800° C. to perform heat treatment (annealing) for threeminutes (T=T7 to T8). As described later, the crystallinity and flatnessof the ZnO layer 11A were further improved by the heat treatment.

After the heat treatment (annealing) of the first ZnO layer (firstlow-temperature grown single-crystal layer) 11A finished, the second ZnOlayer 11B was grown at a higher temperature and a higher pressure thanthose of the first ZnO layer 11A. That is, the substrate temperature waslowered from the heat treatment temperature (800° C.) to 600° C. (secondlow growth temperature) higher than the first low growth temperature.Further, the reaction chamber pressure was increased from 10 kPa (lowgrowth pressure) to 80 kPa (high growth pressure). After the substratetemperature became stable, DMZn was supplied from the shower head ontothe ZnO layer 11A at a flow rate of 10 μmol/min (T=T9 to T10: the growthperiod of the second ZnO layer 11B indicated by the period G1B in FIG.13). At this time, the ratio of the water vapor flow rate to the organicmetal (DMZn) flow rate (F_(H2O)/F_(MO) ratio), the so-called VI/IIratio, was at 64. DMZn was supplied for about 2.5 minutes to form a 40nm thick second ZnO layer (second low-temperature grown single-crystallayer, hereinafter also simply called a second single-crystal layer) 11Bat a growth rate of 16 nm/min.

After the growth of the second ZnO layer 11B finished, the ZnO-basedsemiconductor layer 12 was grown at a higher temperature (high growthtemperature) than that of the second ZnO layer (second single-crystallayer) 11B. That is, the substrate temperature was raised from 600° C.(second low growth temperature) to 800° C. (high growth temperature)with maintaining the reaction chamber pressure at 80 kPa. After thesubstrate temperature became stable, DMZn was supplied from the showerhead onto the ZnO layer 11B at a flow rate of 10 μmol/min (T=T11 to T12:the growth period of the ZnO-based semiconductor layer 12 indicated bythe period G2 in FIG. 13). At this time, the ratio of the water vaporflow rate to the organic metal (DMZn) flow rate (F_(H2O)/F_(MO) ratio),the so-called VI/II ratio, was at 64. DMZn was supplied to the substrate10 for about 60 minutes to form the ZnO-based semiconductor layer (ZnOlayer) 12 about 1 μm thick at a growth rate of 17 nm/min.

Note that the standby time until the substrate temperature becomesstable before the growth of the ZnO-based semiconductor layer 12 can beshared with the heat treatment of the second ZnO layer 11B.

As such, the ZnO-based semiconductor layer 12 (high-temperature grownsingle-crystal layer, hereinafter also called a third single-crystallayer) about 1 μm thick was grown at a higher growth temperature (800°C.) than that of the second ZnO layer 11B at a growth rate of 17 nm/min.In this embodiment, description is made taking as an example the case ofgrowing ZnO as the ZnO-based semiconductor layer 12, but not beinglimited to this, a Mg_(x)Zn_((1-x))O crystal, a ternary crystal, may begrown by using, e.g., Cp2Mg (biscyclopentadienyl magnesium) that is ametalorganic compound not containing an oxygen atom either, as well asDMZn.

After the growth of the ZnO-based semiconductor layer 12 finished, thesubstrate was cooled with keeping water vapor flowing. After thesubstrate temperature became 300° C. or lower, the supply of the watervapor was stopped (T=T13).

As described above, a with-crystal-grown-layer substrate (hereinaftersimply called a with-grown-layer substrate) 15 that is the substratehaving the first ZnO layer (first single-crystal layer) 11A, the secondZnO layer (second single-crystal layer) 11B, and the ZnO-basedsemiconductor layer (third single-crystal layer) 12 grown thereon wasproduced.

[Crystallinity of the First ZnO Layer (First Low-Temperature GrownSingle-Crystal Layer) 11A]

The first ZnO layer (first single-crystal layer) 11A obtained throughthe above growth process (T=T5 to T6) is a single-crystal layerc-axis-oriented on the A-plane ({11-20} plane) sapphire substrate. Thatis, the {0001} plane of a zinc oxide single crystal is laid on asubstrate of an α-sapphire single crystal with the {11-20} plane as themain plane. The single-crystal quality and flatness of the ZnO layer 11Awere examined by RHEED (reflection high energy electron diffraction)measurement and AFM (atomic force microscope) measurement.

FIG. 14A shows an RHEED diffraction image of the ZnO layer 11A withoutthe heat treatment (anneal) after the growth process (T=T5 to T6)finished, that is, just after the growth. As shown in FIG. 14A, an RHEEDdiffraction image having streaks located at equal intervals wasobtained. From this, it was seen that the surface crystal arrangement isa single-crystal arrangement and flat. Also, it was seen from AFM(atomic force microscope) measurement that Rms (root-mean-squareroughness) for a 1 μm² area is 0.62 nm (nanometers). Since the c-axislength (lattice constant) of a ZnO crystal is 5.207 Å, surface roughnessis on the order of the c-axis length of the ZnO crystal. Hence it wasseen that the crystal layer surface is flat. From these results, it wasconfirmed that the ZnO layer 11A is a single-crystal layer good inflatness.

FIG. 14B shows an RHEED diffraction image of the ZnO layer 11A on whichheat treatment was performed at a temperature of 800° C. for threeminutes after the growth. In the present embodiment, the pressure Pa atthe heat treatment was a low pressure (the same as at the crystalgrowth; Pa=Pg=10 kPa). Further, as mentioned above, while H₂O (watervapor) was being supplied to the substrate 10, that is, in a water vaporatmosphere, the heat treatment was performed.

As shown in FIG. 14B, it was ascertained that the RHEED diffractionimage obtained after the heat treatment shows further clear streaks thanthat obtained without the heat treatment (FIG. 14A). Also, it was seenfrom AFM measurement that Rms (root-mean-square roughness) is 0.5 nm orless and less than or equal to the c-axis length of the ZnO crystal.Thus, it was confirmed that the single-crystal quality and flatness wereimproved by the heat treatment. Namely, the first single-crystal layer11A is a thin film, but is a low-temperature grown single-crystal layerwith flatness of the order of sub-nanometers.

As such, it was confirmed that the ZnO layer 11A already had goodflatness and single-crystal quality after the crystal growth finishesand that the single-crystal quality is further improved by the heattreatment with the layer having flatness of less than or equal to thec-axis length of the ZnO crystal.

Although description has been made taking as an example the case ofgrowing the ZnO layers as the first single-crystal layer 11A and thesecond single-crystal layer 11B, they may be another ZnO-based compoundcrystal.

[Growth Conditions of the First Low-Temperature Grown Single-CrystalLayer 11A]

In the present embodiment, description has been made taking as anexample the case of growing the first ZnO layer (first single-crystallayer) 11A using an A-plane sapphire substrate, where the A-plane is the{11-20} plane of an α-sapphire (α-Al₂O₃), as the substrate 10 and DMZnas metalorganic compound material not containing oxygen, at a growthtemperature of 400° C. at a growth pressure of 10 kPa at a growth rateof 1.7 nm/min, to be 25 nm thick.

However, conditions of the substrate, material gas, growth temperature,growth pressure, growth rate, grown-layer thickness, etc., cited in thepresent embodiment are shown only as an example, not being limited tothese. The ZnO layer 11A was grown under various growth conditionsincluding heat treatment conditions, and conditions for growing asingle-crystal layer high in flatness were examined. Growth conditionsof the first ZnO layer (first single-crystal layer) 11A will bedescribed in detail below.

<Substrate>

The A-plane sapphire substrate is most suitable. This is because thelattice mismatch between the A-plane of a sapphire crystal and theC-plane of a ZnO crystal is small. That is, the lattice mismatch betweensapphire <0001> direction (oxygen atoms) and ZnO <11-20> direction (zincatoms) is 0.07%, and the lattice mismatch between sapphire <10-10>direction (oxygen atoms) and ZnO <10-10> direction (zinc atoms) is2.46%. Here, because the ZnO <11-20> direction is locked in alignmentwith the sapphire <0001> direction, a ZnO single crystal can be grownwithout 30° rotated domains being formed in the growth.

Further, as to usable substrates, for the ZnO crystal c-axis oriented ona substrate surface, a crystal substrate of which the substrate surfacehas lattice matching points at rotationally symmetric positions of 60°,120°, and 180° is appropriate, and for the ZnO crystal a-axis or m-axisoriented, a crystal substrate where a lattice matching point exists at arotationally symmetric position of 180° is appropriate. For example, anR-plane sapphire substrate, an M-plane sapphire substrate, a SiC(silicon carbide) substrate, a GaN (gallium nitride) substrate, a Ga₂O₃(gallium oxide) substrate, a Si (silicon) substrate, or the like can beused.

<Growth Temperature>

It is suitable that the growth temperature is a temperature (“low growthtemperature”) lower than a crystal growth temperature (“high growthtemperature”) at which a ZnO single crystal is generally grown. This isbecause at the high growth temperature ZnO tends to grow like islandsand not grow into a single crystal in the form of a layer.

More specifically, the growth temperature is preferably in the range of250° C. to 450° C., more preferably in the range of 300° C. to 400° C.At temperatures of 250° C. and lower, because its migration length isshort, ZnO tends to become amorphous or polycrystalline. Further, asmentioned above, at temperatures of 450° C. and higher, ZnO tends togrow like islands, and thus flatness tends to decrease (the Rms valuebecomes larger).

<Growth Pressure>

The larger migration lengths of reactive chemical species (DMZn, H₂O,intermediate products, Zn atoms before crystallized, O atoms beforecrystallized, and the like) on the substrate surface are preferable forthe single crystal growth. Hence, the reduced pressure growth issuitable. Specifically, a pressure of 1 kPa to 30 kPa is suitable, and apressure of 5 kPa to 20 kPa is more preferable. If the growth pressureis 1 kPa or lower, the growth rate is significantly slow.

<Material Gas>

Forming a ZnO single-crystal layer under low growth pressure and lowgrowth temperature requires the selection of highly reactive materials.This is because, if the reactivity is low, the layer fails tocrystal-grow, or becomes amorphous or polycrystalline. As the Zn source,a metalorganic compound not containing oxygen in its constituentmolecule and highly reactive with oxygen source material is suitable.Other than DMZn mentioned above, for example, DEZn (diethyl zinc) may beused. As the oxygen source, H₂O (water vapor) large in the polarizationwithin the molecule and having high reactivity with the metalorganiccompound material is suitable.

<Growth Rate>

The growth rate is preferably in the range of 0.4 nm/min to 9 nm/min,more preferably in the range of 0.8 nm/min to 4 nm/min. If the growthrate is 9 nm/min or greater, the surface bumps/dips or asperities maybecome large, resulting in insufficient flatness.

<Grown Layer Thickness>

The grown layer thickness should be in the range of 5 nm to 60 nm,preferably in the range of 10 nm to 40 nm, more preferably from 15 nm to30 nm. That is, if the layer thickness is less than 5 nm, the crystallayer may fail to sufficiently cover the substrate surface. If 60 nm orgreater, the surface asperities may become large, resulting ininsufficient flatness.

<VI/II Ratio (F_(H2O)/F_(MO) Ratio)>

The ratio of the water vapor flow rate to the organic metal (DMZn) flowrate (F_(H2O)/F_(MO) ratio) need only be about two or greater.Specifically, the ratio of about 2,000 will suffice. The water vaporflow rate should be up to about 70% of the saturation water vapor amountat which water vapor does not precipitate in the shower head.

<Heat Treatment Conditions>

As described above, the ZnO layer 11A already has good flatness andsingle-crystal quality after the crystal growth finishes, and thesingle-crystal quality and flatness can be further improved by the heattreatment. The heat treatment should be performed under low pressurethat makes the migration length longer. The pressure range suitable forthe heat treatment is the same as that of the growth pressure of the ZnOlayer 11A.

Specifically, it is suitable that the heat treatment temperature for thegrown ZnO layer 11A is from 700° C. to 1,100° C. It is suitable thattreatment time is from 1 to 60 minutes. It is more preferable that thetreatment temperature is from 800° C. to 1,000° C. and that thetreatment time is from 3 to 10 minutes. If the treatment temperature islower than 700° C., the effect is low, and if 1,100° C. or higher, thelayer surface becomes rough. If the treatment time is 60 minutes orlonger, film deficient portions may occur due to film vaporization.

[Growth Conditions of the Second Low-Temperature Grown Single-CrystalLayer 11B] <Growth Temperature>

The growth temperature is preferably from 500° C. to 600° C. to form ZnOin the form of a uniform layer on the ZnO layer 11A. If the growthtemperature is lower than 500° C., crystallinity will not suffice, andthus heat treatment will be necessary. If 650° C. or higher, ZnO willgrow like islands on the ZnO layer 11A.

<Growth Pressure>

For the second ZnO layer (second low-temperature grown single-crystallayer) 11B, the growth pressure of 40 kPa to 120 kPa is suitable. Thisis because the lateral growth (two-dimensional growth mode) is promoted,resulting in the grown layer being flat. Note that this upper limit isthat from the air tightness of the MOCVD apparatus, not from filmforming conditions.

<Grown Layer Thickness>

For the second ZnO layer (second single-crystal layer) 11B, the layerthickness of 5 nm to 80 nm is suitable. If less than 5 nm thick, thelayer will not sufficiently cover the ZnO layer 11A. If greater than 80nm thick, hillocks will occur. The layer thickness is more preferablyfrom 20 nm to 60 nm.

<Growth Rate>

The growth rate should be 60 nm/min or less, preferably in the range of5 nm/min to 60 nm/min. This is for preventing the occurrence of abnormalgrowth.

[Growth Conditions of the ZnO-Based Semiconductor Layer(High-Temperature Grown Single-Crystal Layer) 12]

The ZnO-based semiconductor layer (high-temperature grown single-crystallayer, also called a third single-crystal layer) 12 is grown on thesecond ZnO layer (second single-crystal layer) 11B having flatness ofthe order of sub-nanometers and excellent in single-crystal qualityusing conditions for growth in a two-dimensional crystal growth mode (orlateral growth mode).

In the present embodiment, description has been made taking as anexample the case of growing the Mg_(x)Zn_((1-x))O crystal layer(high-temperature grown single-crystal layer) 12 on the ZnO layer 11Busing water vapor and metalorganic compound material (DMZn) notcontaining oxygen, at the high growth temperature (800° C.) at a growthpressure of 80 kPa higher than that of the ZnO layer 11A, like for theZnO layer 11B, at a growth rate of 17 nm/min to be 1 μm thick.

However, conditions for growing the ZnO-based semiconductor layer 12 onthe ZnO layer 11B cited in the present embodiment are shown only as anexample, not being limited to these. The ZnO-based semiconductor layerwas grown under various growth conditions, and conditions for growingthe ZnO-based semiconductor layer 12 excellent in flatness andsingle-crystal quality were examined. Growth conditions for growing theZnO-based semiconductor layer (third single-crystal layer) 12 on thesecond ZnO layer (second single-crystal layer) 11B will be described indetail below.

<Growth Pressure>

It was found that as the growth pressure increases, the crystallinity isimproved. That is, it was found that as the growth pressure increases,the lateral growth (two-dimensional growth mode) is promoted, resultingin the plane (C-plane) orthogonal to the c-axis being flat. It was foundthat in order to obtain a flat crystal growth surface free of bumps/dipsor pits as the surface of the ZnO-based semiconductor layer(high-temperature grown single-crystal layer) with good crystallinity,high growth pressure is desirable. Specifically, the growth pressure ispreferably 40 kPa or higher, more preferably 60 kPa or higher, furthermore preferably 80 kPa or higher. The upper limit should be about 120kPa. Note that this upper limit is that from the airtightness of theMOCVD apparatus, not from film forming conditions.

<Growth Temperature>

It was found that as the growth temperature increases, dislocationdensity decreases, and thus the crystallinity and flatness are improved.Specifically, the growth temperature should be 700° C. or higher atwhich a flat surface is formed in a c-axis direction. The upper limit isabout 850° C. at which the layer can hardly grow when using H₂O (watervapor). The growth temperature is preferably in the range of 740° C. to810° C., most preferably in the range of 780° C. to 810° C.

<Grown Layer Thickness>

The surface morphology, flatness and crystallinity were evaluated withSEM (scanning electron microscope) and TEM (transmission electronmicroscope). FIG. 15 shows a cross-sectional TEM bright field image(layer thickness: 1.05 μm) after the growth of the ZnO-basedsemiconductor layer (third single-crystal layer) 12 and surface SEMimages (SEM-1 to SEM-4) of grown layers of different layer thicknesses.The SEM images SEM-3 and SEM-4 were taken so as to include a minutecrystal adsorbed to the surface in the view field on purpose as seen inthe SEM image in order to prove that the surface was in focus. Note thatviewing the whole, the occurrence of an adsorbed minute crystal is rare.

For layer thicknesses (t) of 0.06 μm (SEM-1) and 0.13 μm (SEM-2), pitdensities are 1.8×10⁹ cm⁻² and 3.7×10⁸ cm⁻², respectively. Dislocationsoriginating from the substrate interface decrease in number withincrease in layer thickness. For a layer thickness of 0.5 μm (SEM-3),hollows of pits have vanished with dislocations converging. That is, thevanishment of hollows of pits is related to the convergence ofdislocations. As such, it was found that the ZnO-based semiconductorlayer 12 formed on the second ZnO layer (second single-crystal layer)11B grows substantially in the form of a layer from the beginning of thegrowth, and that as the layer thickness increases,threading-dislocations originating from the substrate interfaceremarkably decrease in number. In addition, it was found that at anearly stage of the growth when the layer thickness is 0.5 μm, the grownlayer becomes completely flat with dislocation density remarkablyreduced. As seen from the SEM image (SEM-4) for the grown layer of alayer thickness (t) of 1.05 μm, the layer subsequently grows withmaintaining flatness. Thus, the grown layer thickness is preferably 0.5μm or greater.

In contrast, in the case of growing the ZnO-based semiconductor layer 12on the first ZnO layer 11A without forming the second ZnO layer 11B, inorder to obtain high flatness, the layer thickness is preferably 1.5 μmor greater, but by providing the second ZnO layer 11B, the layerthickness of the ZnO-based semiconductor layer to obtain high flatnesscan be reduced. In addition, the ZnO-based semiconductor layer 12 ofgood crystallinity with a smaller amount of threading-dislocations canbe formed.

<Growth Rate>

The growth rate is preferably in the range of 5 to 60 nm/min. If thegrowth rate is 60 nm/min or greater, abnormal growth is likely to occur.

<Crystal Composition>

For example, a Mg_(x)Zn_((1-x))O (0≦x≦0.43) crystal can be used as theZnO-based semiconductor layer 12. However, as Mg composition xincreases, the difference in lattice constant in an a-axis directionincreases, resulting in an increase in the defect density of the formedsemiconductor crystal layer. Hence, it is more preferable that 0≦x≦0.3.

As mentioned above, the ZnO-based semiconductor layer 12 may be anotherZnO-based compound crystal. For example, it may be a ZnO-based compoundcrystal having some of Zn (zinc) replaced by Ca, or a ZnO-based compoundcrystal having some of O (oxygen) replaced by Se, S, Te, or the like.

<Material Gas>

When grown at a high temperature, a ZnO crystal of high quality can begrown because the migration lengths of reactive chemical species in thecrystal growth surface are sufficient. On the other hand, gas materialthat is an oxygen source becomes hard to adsorb to the substratesurface, thus hindering the growth. As the oxygen source, H₂O (watervapor) that is large in the polarization within the molecule, thusadsorbing even at high temperatures to the substrate surface issuitable.

As the Zn source, a metalorganic compound not containing oxygen andhighly reactive with oxygen source material is suitable. Other than DMZnmentioned above, for example, DEZn (diethyl zinc) may be used. As the Mgsource, Cp2Mg (biscyclopentadienyl magnesium) can be used.

<Dopants>

In order to adjust the conductivity type (n-type) of the ZnO-basedsemiconductor layer 12, one or more of TMGa (trimethyl gallium), TEGa(triethyl gallium), TMAl (trimethyl aluminum), TMIn (trimethyl indium),and TEIn (triethyl indium) need to be added.

They are preferably doped during the last half of the growth time periodin which the crystal plane becomes flat. Specifically, they arepreferably doped after the layer grows 0.25 μm thick. That is, it ispreferable that the ZnO-based semiconductor layer 12 is an undoped layerat least 0.25 μm thick.

<Growth Sequence>

FIG. 16 shows a crystal growth sequence of a modification of the presentembodiment. In this modification, the second ZnO layer (secondsingle-crystal layer) 11B continues to be grown until the ZnO-basedsemiconductor layer (ZnO layer) 12 starts to be grown. The other growthconditions are the same as in the above embodiment.

More specifically, in the above-described embodiment, before the growthtemperature was raised from 600° C. (second low growth temperature) to800° C. (high growth temperature), the growth of the second ZnO layer(second single-crystal layer) 11B is finished (growth period T=T9 toT10, indicated by period G1B in FIG. 13). In this modification, DMZncontinues to be supplied until the ZnO-based semiconductor layer 12starts to be grown (T=T11), and the growth period of the secondsingle-crystal layer 11B is T9 to T11 (period G1B′ in FIG. 16). Thetemperature rise time is preferably within 10 minutes in terms ofpreventing hillocks from occurring, and so on.

FIG. 17 shows the full width at half maximum (FWHM) of the X-raydiffraction (0002) ω and (10-10) ω rocking curves for the case ofgrowing the ZnO-based semiconductor layer (third single-crystal layer)12 on the first ZnO layer 11A (LT1), the case of growing the ZnO-basedsemiconductor layer 12 on the second ZnO layer 11B grown in period T=T9to T10 (period G1B in FIG. 13) (LT2A), and the case of growing theZnO-based semiconductor layer 12 on the second ZnO layer 11B grown inperiod T=T9 to T11 (period G1B′ in FIG. 16) (LT2B).

It can be seen that the layers 12 in the cases of growing the ZnO-basedsemiconductor layer 12 on the second ZnO layer 11B (LT2A, LT2B) aresuperior in crystallinity, and that the layer 12 in the case where thesecond ZnO layer 11B continued to be grown until the ZnO-basedsemiconductor layer 12 started to be grown (LT2B) is further superior incrystallinity.

As described above in detail, by using the above growth conditions, theZnO-based semiconductor layer 12 having high flatness and crystallinitycan be grown on the ZnO layer (second single-crystal layer) 11B that isthin but excellent in flatness and single-crystal quality. That is, theZnO-based semiconductor layer (third single-crystal layer) 12 can begrown to have enough thickness to be applied to device manufacture withhaving further high flatness and crystallinity, and hence can be appliedwidely to device manufacture. For example, a light emitting layer(active layer), an LED light emitting operation layer, a clad layer ofsemiconductor laser devices, or a device operation layer of electronicdevices or the like can be grown directly on the ZnO-based semiconductorlayer 12. Or, the ZnO-based semiconductor layer 12 grown under the abovegrowth conditions may be arranged to form part of an LED light emittingoperation layer, a clad layer, or a device operation layer.

[With-Grown-Layer Substrate 15]

The with-grown-layer substrate 15 obtained through the above processescan be subsequently used in the MOCVD apparatus to manufacturesemiconductor devices without being cooled. Or, after cooled,semiconductor devices may be manufactured using the MOCVD apparatus oranother crystal growing apparatus.

That is, with the with-grown-layer substrate 15, a single crystal can begrown directly on the single-crystal layer (ZnO-based semiconductorlayer 12) excellent in single-crystal quality and flatness using theMOCVD apparatus or another crystal growing apparatus. Thus, a highquality ZnO-based semiconductor layer having a small number of crystaldefects and excellent in single-crystal quality and flatness can beformed.

Further, with the with-grown-layer substrate 15, optical semiconductordevices, various electronic devices, and the like can be formed byvarious methods other than MOCVD such as MBE, plasma CVD, PLD (PulsedLaser Deposition), and hydride VPE.

As such, according to the present invention, there is provided awith-grown-layer substrate having formed thereon high qualitysingle-crystal layers having a small number of crystal defects and lowdefect density, and excellent in single-crystal quality and flatnesswhich is applicable to the manufacture of optical semiconductor devicesand various electronic devices.

Embodiment 4

FIG. 18 is a cross-sectional view of a semiconductor light emittingdevice structure grown on the substrate 10 according to the presentinvention. More specifically, a light emitting operation layer(hereinafter also called an LED operation layer) 20 consisting of ann-type ZnO-based semiconductor layer 21, a ZnO-based semiconductoractive layer 22, and a p-type ZnO-based semiconductor layer 23 wasformed on the with-grown-layer substrate 15 having the first ZnO layer(first single-crystal layer) 11A, a second ZnO layer (secondsingle-crystal layer) 11B, and the ZnO-based semiconductor layer 12formed thereon according to the above embodiment. Crystal growthconditions of the LED operation layer 20 were the same as those of theZnO-based semiconductor layer 12 in the above embodiment unlessotherwise noted. That is, the crystal growth sequence was the same asthat shown in FIG. 13, and using the same growth temperature, growthpressure, material gas, etc., as in the period (G2) of T=T11 to T12, then-type ZnO-based semiconductor layer 21, ZnO-based semiconductor activelayer 22, and p-type ZnO-based semiconductor layer 23 were sequentiallyformed on the ZnO-based semiconductor layer 12. Growth conditions suchas growth temperature and growth pressure of the LED operation layer 20may not necessarily be the same as those of the ZnO-based semiconductorlayer 12. That is, crystal growth conditions of the LED operation layer20 are preferably within the ranges of those of the ZnO-basedsemiconductor layer 12.

As such, by growing an LED operation layer on the ZnO-basedsemiconductor layer 12 having good flatness and single-crystal qualityunder growth conditions similar to those of the ZnO-based semiconductorlayer 12, the LED operation layer flat and excellent in crystallinitycan be formed. Description will be made below taking as an example thecase of growing Mg_(x)Zn_((1-x))O as the ZnO-based semiconductor.

After an undoped ZnO-based semiconductor layer 12 was grown, withmaintaining the growth temperature and growth pressure (800° C., 80 kPa)the n-type ZnO-based semiconductor layer 21 was grown. The undopedZnO-based semiconductor layer 12 is preferably at least 0.25 μm thick asmentioned above.

Keeping the H₂O (water vapor) flow rate at 640 μmol/min and increasingthe DMZn flow rate from 10 μmol/min to 30 μmol/min, a Ga-dopedMg_(x)Zn_((1-x))O crystal 3 μm thick was grown. The flow rate of Mgmaterial gas (Cp2Mg) should be adjusted according to Mg crystalcomposition x.

In order to control the conductivity type (n-type), TEGa was dopedduring the Mg_(x)Zn_((1-x))O crystal growth so that its concentration inthe Mg_(x)Zn_((1-x))O crystal became 5×10¹⁸ cm⁻³.

Then, the DMZn flow rate was decreased to 1 μmol/min, and the activelayer 22 (Mg_(x)Zn_((1-x))O crystal) 30 nm thick was grown. Here, bydecreasing the DMZn flow rate, the ratio of the H₂O (water vapor) flowrate to the DMZn flow rate (VI/II ratio) was increased from 21 to 640.Thereby, deficiency of oxygen or the like in the grown layer could bereduced, and thus high light-emission efficiency could be obtained.

Then, the p-type ZnO-based semiconductor layer 23 was grown.Specifically, at a DMZn flow rate of 1 μmol/min, an N (nitrogen)-dopedMg_(x)Zn_((1-x))O crystal 100 nm thick was grown. Here, during theMg_(x)Zn_((1-x))O crystal growth, NH₃ (ammonia) was supplied as p-typeimpurity material (dopant) at a flow rate of 180 μmol/min so thatnitrogen impurity concentration Na (N) became 8×10¹⁹ cm⁻³.

After the above process finished, the pressure was maintained at 80 kPawith keeping water vapor flowing until the substrate temperature became300° C. After the substrate temperature became 300° C. or lower, watervapor was stopped, and the substrate was taken out of the reactionchamber after the substrate temperature became room temperature.

The case of using Mg_(x)Zn_((1-x))O crystals as the n-type ZnO-basedsemiconductor layer 21, active layer 22, and p-type ZnO-basedsemiconductor layer 23 has been described.

In this case,n-type ZnO-based semiconductor layer 21: Mg_(x)Zn_((1-x))O (0≦x≦0.43),p-type ZnO-based semiconductor layer 23: Mg_(x)Zn_((1-x))O (0≦x≦0.43),andactive layer 22: Mg_(x)Zn_((1-x))O/Mg_(y)Zn_((1-y))O (0≦(x, y)≦0.43;y<x) can be used.

Here, Mg crystal composition x should be 0.63 or less for a layerthickness of about 0.5 μm or less and 0.43 or less for 0.5 μm orgreater. This is because if the Mg composition is higher, the phaseseparation of MgO occurs in the MgZnO crystal.

Each of the n-type ZnO-based semiconductor layer 21, the active layer22, and the p-type ZnO-based semiconductor layer 23 may be of amultilayered structure depending on the light emitting devicecharacteristic. Further, the active layer 22 may be of an MQW(Multi-quantum well) structure. Especially in the case of an MQW activelayer, variation in the thickness of crystal layers (a well layer, abarrier layer) varies quantum state energy, quantum state density, andthe like, thus greatly affecting the emission wavelength, internalquantum efficiency, and the like, and hence the effect produced by usingthe ZnO-based semiconductor layer 12 excellent in flatness andsingle-crystal quality is further remarkable.

A with-LED-operation-layer substrate 25 that was manufactured throughthe above-described process was evaluated with a differentialinterference microscope and a scanning electron microscope (SEM). FIG.19A is a differential interference microscope image of the surface ofthe with-LED-operation-layer substrate 25 produced through theabove-described process. For comparison, FIG. 19B shows a differentialinterference microscope image of the surface of the substrate 25 havingthe LED operation layer formed at a growth temperature (Tg) of 680° C.From these results, it was ascertained that the surface of thewith-LED-operation-layer substrate 25 produced through the above processwas free of bumps/dips or pits and was an extremely flat mirror surfacein macroscopic to microscopic views.

[Manufacture of Semiconductor Light Emitting Devices]

Semiconductor light emitting devices (LEDs) were manufactured using thewith-LED-operation-layer substrate 25 produced through the aboveprocess, through the same processes as in Embodiment 2. FIG. 20 is a topplan view of a semiconductor light emitting device (LED) 30, and FIG. 21is a cross-sectional view of LEDs 30. FIG. 21 shows that two LEDs 30 anda device partition groove 32 for separating these by breaking areformed.

Specifically, using photolithography, EB (electron beam) vapordeposition, and so on, a p-side electrode 33, a p-side connectionelectrode 34, and an n-side connection electrode 35 were formed.Processes such as etching, heat treatment, back side polishing, and theformation of scribe grooves were the same as in Embodiment 2. Also,processes such as the breaking (dividing into chips) of the wafer formedin this way were the same as in Embodiment 2.

Because the surface of the with-LED-operation-layer substrate 25 of thepresent invention was free of bumps/dips, the bottom of the devicepartition groove 32 was also formed extremely flat, and pressure couldbe accurately applied to the partition groove bottom with a knife edge37. Thus, when applying pressure to cleave, stress was uniformlyapplied, and hence the occurrences of cleavage failure due to devicesection chipping, the deviation of a cleaved surface, etc., could bereduced, improving the cleavage yield. Further, without a potentialcrystal grain boundary (domain) in crystal planes, when cleaving, chipsin a crystal layer due to a grain boundary decreased in number,improving the separation yield.

When devices were formed using the with-LED-operation-layer substrate 25of the present invention, a resist pattern formation yield was about98%. Also, the cleavage yield was extremely good, about 98%.

As described above, because an LED operation layer is grown on theZnO-based semiconductor layer 12 that is a high quality single crystallayer having a small number of crystal defects and excellent insingle-crystal quality and flatness, the LED operation layer flat andexcellent in crystallinity can be formed, and high performance LEDshaving leak current and current concentration suppressed and thus highin light-emission efficiency and excellent in device lifetime can bemanufactured. Further, high process accuracy in semiconductor processessuch as lithography and etching can be obtained, and the productionyield in cleavage, breaking, and like is also high.

In the above embodiments, description has been made taking as an examplethe case of applying the present invention to a semiconductor lightemitting device (LED), but not being limited to this, the presentinvention can be applied to optical semiconductor devices and electronicdevices which can be formed using a ZnO-based semiconductor layer, suchas laser diodes (LD), surface acoustic wave devices, and MOSFETs.

As described above in detail, according to the present invention, usingmetalorganic compound material not containing oxygen together with H₂O(water vapor) having high reactivity with the metalorganic compoundmaterial, a zinc oxide single-crystal layer (first single-crystal layer)can be grown at a low growth temperature and a low growth pressure on asubstrate of another material such as an A-plane sapphire substrate.Further, by performing heat treatment under low pressure (reducedpressure) in a water vapor atmosphere, the single-crystal quality andflatness of the grown layer can be further improved. In addition, bygrowing a single-crystal layer (second single-crystal layer) of azinc-oxide-based compound semiconductor on the first single-crystallayer at a low growth temperature, its single-crystal quality andflatness can be improved. The single-crystal layer is excellent inflatness and single-crystal quality and also has a small number ofcrystal defects and low defect density. Moreover, because the MOCVDmethod is used, large area growth and multi-wafer growth are possible.Thus, the present method is excellent in mass productivity andproduction costs.

Furthermore, the ZnO-based compound semiconductor layer having furtherhigh flatness and crystallinity can be grown on the secondsingle-crystal layer excellent in flatness and single-crystal quality.The ZnO-based compound semiconductor layer (third single-crystal layer)12 can be grown to have enough thickness to be applied to devicemanufacture as well as high flatness and crystallinity. Therefore, anLED light emitting operation layer, a clad layer of semiconductor laserdevices, a device layer of electronic devices, or the like can be growndirectly on the ZnO-based semiconductor layer 12.

By growing an LED operation layer on the ZnO-based semiconductor layer12, the LED operation layer flat and excellent in crystallinity can beformed, and high output power, high performance LEDs having leak currentand current concentration suppressed and thus high in light-emissionefficiency can be provided. Further, LEDs excellent in device lifetimecan be manufactured. Yet further, high accuracy in semiconductorprocesses is obtained, and the production yield in cleavage, breaking,and like is also high.

The invention has been described with reference to the preferredembodiments thereof. It should be understood by those skilled in the artthat a variety of alterations and modifications may be made from theembodiments described above. It is therefore contemplated that theappended claims encompass all such alterations and modifications.

This application is based on Japanese Patent Applications P2008-236922and P2008-236923 which are hereby incorporated by reference.

1. A method of growing a zinc-oxide-based semiconductor layer on asubstrate by an MOCVD method, comprising the steps of: using water vaporand a metalorganic compound not containing oxygen, (a) performingcrystal growth at a low growth temperature and at a low growth pressurein the range of 1 kPa to 30 kPa to form a low-temperature grownsingle-crystal layer; and (b) performing crystal growth at a high growthtemperature and at a pressure higher than said low growth pressure toform a high-temperature grown single-crystal layer on saidlow-temperature grown single-crystal layer.
 2. A method according toclaim 1, wherein said low growth temperature is a temperature in therange of 250° C. to 450° C.
 3. A method according to claim 1, whereinsaid high growth temperature is a temperature in the range of 700° C. to850° C.
 4. A method according to claim 1, wherein said pressure higherthan said low growth pressure is a pressure in the range of 40 kPa to120 kPa.
 5. A method according to claim 1, wherein said high-temperaturegrown single-crystal layer forming step performs heat treatment on saidlow-temperature grown single-crystal layer before the growth of saidhigh-temperature grown single-crystal layer.
 6. A method according toclaim 1, wherein said substrate is an α-sapphire single crystal, and acrystal growth plane is a {11-20} plane.
 7. A method according to claim1, wherein said low-temperature grown single-crystal layer is a zincoxide (ZnO) layer, and said high-temperature grown single-crystal layeris a Mg_(x)Zn_((1-x))O (0≦x≦0.43) layer.
 8. A method according to claim1, wherein the layer thickness of said low-temperature grownsingle-crystal layer is in the range of 5 nm to 60 nm.
 9. A methodaccording to claim 1, wherein the layer thickness of saidhigh-temperature grown single-crystal layer is 1.5 μm or greater.
 10. Amethod according to claim 8, wherein the growth rate of saidlow-temperature grown single-crystal layer is 9 nm/min or less.
 11. Amethod according to claim 9, wherein the growth rate of saidhigh-temperature grown single-crystal layer is 60 nm/min or less.
 12. Amethod according to claim 7, wherein in said high-temperature grownsingle-crystal layer forming step, an impurity is doped after thethickness of the grown layer becomes greater than 1 μm.
 13. A methodaccording to claim 1, wherein said substrate is one of sapphire, galliumoxide (Ga₂O₃), silicon carbide (SiC), gallium nitride (GaN), and siliconsubstrates.
 14. A method of manufacturing a semiconductor light emittingdevice by growing zinc-oxide-based semiconductor layers on a substrateby an MOCVD method, comprising the steps of: using water vapor and ametalorganic compound not containing oxygen, (a) performing crystalgrowth at a low growth temperature and at a low growth pressure in therange of 1 kPa to 30 kPa to form a low-temperature grown single-crystallayer; (b) performing crystal growth at a high growth temperature and ata pressure higher than said low growth pressure to form ahigh-temperature grown single-crystal layer on said low-temperaturegrown single-crystal layer; and (c) performing crystal growth at a highgrowth temperature and at a pressure higher than said low growthpressure to form a light emitting layer on said high-temperature grownsingle-crystal layer.
 15. A method according to claim 14, wherein saidlow growth temperature is a temperature in the range of 250° C. to 450°C.
 16. A method according to claim 14, wherein said high growthtemperature is a temperature in the range of 700° C. to 850° C.
 17. Amethod according to claim 14, wherein said pressure higher than said lowgrowth pressure is a pressure in the range of 40 kPa to 120 kPa.
 18. Amethod according to claim 14, wherein said high-temperature grownsingle-crystal layer includes an undoped layer of at least 1.0 μmthickness formed on said low-temperature grown single-crystal layer. 19.A method according to claim 14, wherein said substrate is an A-planesapphire substrate, and said low-temperature grown single-crystal layerand said high-temperature grown single-crystal layer are each aMg_(x)Zn_((1-x))O (0≦x≦0.43) layer.
 20. A method according to claim 1,wherein said low-temperature grown single-crystal layer forming step (a)comprises the steps of: (a1) performing crystal growth at a first lowgrowth temperature and at a low growth pressure in the range of 1 kPa to30 kPa to form a first low-temperature grown single-crystal layer; and(a2) performing crystal growth at a second low growth temperature higherthan said first low growth temperature and at a pressure higher thansaid low growth pressure to form a second low-temperature grownsingle-crystal layer on said first low-temperature grown single-crystallayer.
 21. A method according to claim 20, wherein said first low growthtemperature is a temperature in the range of 250° C. to 450° C., andsaid second low growth temperature is a temperature in the range of 500°C. to 650° C.
 22. A method according to claim 20, wherein said firstlow-temperature grown single-crystal layer and said secondlow-temperature grown single-crystal layer are each a zinc oxide (ZnO)layer, and said high-temperature grown single-crystal layer is aMg_(x)Zn_((1-x))O (0≦x≦0.43) layer.
 23. A method according to claim 20,wherein the layer thickness of said first low-temperature grownsingle-crystal layer is in the range of 5 nm to 60 nm.
 24. A methodaccording to claim 20, wherein the layer thickness of said secondlow-temperature grown single-crystal layer is in the range of 5 nm to 80nm.
 25. A method according to claim 20, wherein the layer thickness ofsaid high-temperature grown single-crystal layer greater than or equalto 0.5 μm.
 26. A method according to claim 20, wherein the growth rateof said second low-temperature grown single-crystal layer and saidhigh-temperature grown single-crystal layer is 60 nm/min or less.
 27. Amethod according to claim 20, wherein the step (a2) of forming saidsecond low-temperature grown single-crystal layer includes a step of:forming said second low-temperature grown single-crystal layer whilechanging the growth temperature from said second low growth temperatureto said high growth temperature, upon transition to the step (b) offorming said high-temperature grown single-crystal layer.
 28. A methodaccording to claim 14, wherein said low-temperature grown single-crystallayer forming step (a) comprises the steps of: (a1) performing crystalgrowth at a first low growth temperature in the range of 250° C. to 450°C. and at a low growth pressure in the range of 1 kPa to 30 kPa to forma first low-temperature grown single-crystal layer; and (a2) performingcrystal growth at a second low growth temperature higher than said firstlow growth temperature and at a pressure higher than said low growthpressure to form a second low-temperature grown single-crystal layer onsaid first low-temperature grown single-crystal layer.
 29. A methodaccording to claim 28, wherein said first low growth temperature is atemperature in the range of 250° C. to 450° C., and said second lowgrowth temperature is a temperature in the range of 500° C. to 650° C.30. A method according to claim 28, wherein said first low-temperaturegrown single-crystal layer and said second low-temperature grownsingle-crystal layer are each a zinc oxide (ZnO) layer, and saidhigh-temperature grown single-crystal layer is a Mg_(x)Zn_((1-x))O(0≦x≦0.43) layer.
 31. A method according to claim 28, wherein saidhigh-temperature grown single-crystal layer includes an undoped layer ofat least 0.25 μm thickness formed on said high-temperature grownsingle-crystal layer.